Isocyanates carry the highly reactive –N=C=O group that links with amines and polyols to form polyurea and polyurethane. Choose aromatic grades for toughness and cost, aliphatic grades for color and UV stability.
An isocyanate is any molecule carrying one or more –N=C=O groups. That group is electron-hungry: it reacts readily with anything that has an active hydrogen, including amines, alcohols and even water. Coatings put that reactivity to work by pairing a diisocyanate or polyisocyanate A-side with an amine or polyol B-side, building long chains and crosslinked networks within seconds to hours.
Isocyanates fall into two camps. Aromatic isocyanates such as MDI and TDI have the NCO group attached to a benzene ring. They are fast, tough and economical, but the ring absorbs UV light, so films yellow and chalk outdoors. Aliphatic and cycloaliphatic isocyanates such as HDI, IPDI and H₁₂MDI have no aromatic ring next to the NCO group, so they hold color and gloss in sunlight. They cost more and react more slowly.
Monomeric diisocyanates are rarely used on their own in field coatings. Manufacturers convert them into prepolymers, trimers and biurets that keep the reactivity while greatly reducing the amount of volatile free monomer.
From the MDI in a spray polyurea A-side to the HDI trimer in a polyaspartic topcoat. Click any CAS number to copy it.
Used forBuilding block for polyurea and polyurethane prepolymers, elastomers and adhesives.
A solid at room temperature (melts near 40 °C), so coatings use it modified, blended or pre-reacted into prepolymers.
Full profileUsed forRigid foam, binders, and aromatic polyurea and polyurethane A-sides.
Dark brown liquid with an average functionality around 2.7; far lower vapor pressure than TDI.
Full profileUsed forThe A-side of most aromatic spray polyurea and hybrid systems.
Pre-reacting part of the MDI with a long-chain polyol lowers vapor pressure and tunes viscosity, flexibility and mix ratio.
Full profileUsed forFlexible foam; TDI adducts in some wood and industrial finishes.
Much higher vapor pressure than MDI, so it is rarely used as free monomer in field-applied coatings.
Full profileUsed forRaw material for the HDI trimers and biurets used in UV-stable topcoats.
Volatile as a monomer, so coatings use low-monomer oligomers (trimer, biuret) instead.
Full profileUsed forHardener for polyaspartic and 2K aliphatic polyurethane topcoats.
The ring-shaped isocyanurate core delivers excellent gloss retention and weathering.
Full profileUsed forHardener for light-stable polyurethane enamels and clearcoats.
A long-time workhorse alongside the trimer, valued for flexibility and weathering.
Full profileUsed forLight-stable prepolymers, aliphatic polyurea and polyurethane dispersions.
Its two NCO groups differ in reactivity, which helps build low-viscosity, low-monomer prepolymers.
Full profileUsed forAliphatic elastomers, light-stable polyurea and optical-grade polyurethanes.
The saturated, non-yellowing cousin of MDI.
Full profileThe carbon in the NCO group sits between nitrogen and oxygen, which both pull electron density away from it. Molecules with an active hydrogen add across the N=C bond:
Formulators express reactivity as % NCO and balance it against the B-side’s equivalent weight using an index, typically run slightly above 100 to make up for stray moisture.
R–NCO + H₂N–R′ → R–NH–CO–NH–R′Urea · secondsR–NCO + HO–R′ → R–NH–CO–O–R′Urethane · minutes to hours, catalyzedR–NCO + H₂O → R–NH₂ + CO₂↑Side reaction · causes bubblesR and R′ stand for the rest of each molecule.
The single biggest choice in an isocyanate-cured coating.
| Property | Aromatic (MDI, TDI) | Aliphatic (HDI, IPDI, H₁₂MDI) |
|---|---|---|
| UV / color stability | Yellows and chalks in sunlight | Excellent gloss and color retention |
| Reactivity | Fast | Slower; often catalyzed |
| Cost | Lower | Higher |
| Typical role | Spray polyurea, foams, primers, basecoats | Topcoats, polyaspartics, clearcoats |
| Common form | Prepolymers, polymeric MDI | Trimers, biurets, prepolymers |
Diisocyanates are respiratory and skin sensitizers. Once a worker is sensitized, even tiny exposures can trigger asthma-like reactions, so prevention is everything.
Always follow the product’s Safety Data Sheet, your employer’s safety program and local regulations. This page is general reference information, not a substitute for them.
Aromatic isocyanates (MDI, TDI) have the NCO group attached to a benzene ring. They are fast and economical but yellow in sunlight. Aliphatic isocyanates (HDI, IPDI, H₁₂MDI) have no aromatic ring, so they resist UV and hold their color, which is why they are used in topcoats and polyaspartics.
It is the weight percentage of reactive isocyanate groups. Pure 4,4′-MDI is about 33.6% NCO, while a typical spray polyurea A-side prepolymer runs around 15–16%. It is used with the B-side equivalent weight to set the mix ratio.
Isocyanate reacts with water to form an amine and carbon dioxide gas. Trapped CO₂ creates bubbles, pinholes and foaming, so substrates, solvents and pigments must be dry.
Pre-reacting MDI with a polyol lowers its vapor pressure, keeps it liquid and easy to process, and lets formulators tune flexibility and mix ratio.